This invention relates to a water-soluble coating composition having excellent physical properties including corrosion resistance, adhesion, impact resistance and so on.
Heretofore, as raw materials for a water-soluble coating composition there have been used natural oils. However, recently, use of natural oils for the above purposes has been regarded to be disadvantageous because their supply is precarious and they cause pollution in respect of smell and smoke. Therefore, instead of natural oils a butadiene low polymer has been given much consideration as a film forming material for a water-soluble coating composition since the coating film formed from a butadiene low polymer or copolymer has very excellent film properties in respect of chemical resistance, solvent resistance, water resistance, corrosion resistance, levelling property and so forth as compared with those prepared from the conventional film forming materials such as natural oils.
In order to prepare a film forming material for a water-soluble coating composition, it is necessary to introduce hydrophilic groups such as carboxyl, hydroxyl, ether, amino and sulfonic acid groups into the molecular structure of the material.
With regard to the introduction of the carboxyl group in to the butadiene lower polymer or copolymer, it is well known that an adduct can be prepared by heating it with .alpha., .beta.-ethylenically unsaturated dicarboxylic acid compound. The addition reaction between butadiene lower polymer or copolymer and .alpha., .beta.-ethylenically unsaturated dicarboxylic acid is carried out through a radical or thermal reaction. Both reactions are accompanied by cross linking reactions among molecules of starting polymer so that gelation of the adduct occurs, which is disadvantageous.
In order to eliminate the above noted difficulties there are known many methods for preparing such an adduct by the addition of certain compounds such as hydroquinone, 2, 6-di-tert-butyl-4-methyl phenol, urethane compounds or metallic salts to the thermal reaction system. According to these methods the amount of addition of .alpha., .beta.-ethylenically unsaturated dicarboxylic acid compounds is very small, the adduct is greatly colored, the viscosity of the adduct is too high to be treated and the molecular weight of the usable starting polymer is limited to a very low range so that the adduct produced by the above conventional methods has insufficient properties as a film forming material for a water-soluble coating composition.
With these difficulties in view, the present inventors made extensive studies on methods for preventing various undesirable side reactions such as the above noted gelation from the addition reaction between a butadiene lower polymer or copolymer and .alpha., .beta.-ethylenically unsaturated carboxylic acid and as the result found that some compounds have excellent effects for the above noted purposes. Japanese patent applications for the above inventions were filed (Japanese patent application Nos. 47-126461 and 48-57676). According to these inventions accomplished by the present inventors the undesirable side reaction including the gelation of the addition reaction can be completely prevented without causing any ill effects to the reaction and further an adduct with a large amount of addition can be obtained from starting polymer having a high molecular weight.
It is an object of the present invention to provide a water-soluble coating composition which will eliminate or alleviate the foregoing disadvantages encountered with the prior-art coating compositions.
It is a further object of this invention to provide a water-soluble coating composition having excellent properties in respect of corrosion resistance, adhesion, impact resistance and so on.
These objects and other features of this invention will be apparent from the following description taken in connection with certain embodiments of the invention.